The present invention relates generally to optical phase locked coherent receivers, and more particularly, to a virtual coherent signal controlled laser oscillator for use in optical phase locked coherent receivers.
Conventional optical phase locked coherent receivers have used laser signal controlled oscillators. It has been determined that the FM response of available laser signal controlled oscillators is undesirable and is unsuitable for phase locking of the optical phase locked coherent receiver in which it is used.
It is therefore an objective of the present invention to provide for a virtual coherent signal controlled laser oscillator for use in optical phase locked coherent receivers.
The present invention provides for a virtual coherent signal controlled laser oscillator for use in optical phase locked coherent receivers. The present invention is intended for use in coherent optical transmitter and receiver systems. In a receiver, the signal controlled laser oscillator may be used as a phase locked laser oscillator, a key element of a coherent optical detector. The shortcoming of currently available signal controlled laser oscillators prevent their use in this application since their response to AC control signals are unsuitable for operation of the phase lock loop. The present invention circumvents this problem by using an implementation that is a combination of a signal controlled microwave oscillator and a fixed laser oscillator.
The present invention has suitable signal controlled (FM) response for use in the optical phase locked coherent receiver, whereas a conventional laser signal controlled oscillator FM response is unsuitable and prevents phase locking.
An exemplary signal controlled laser oscillator comprises a signal controlled laser oscillator that receives a fixed bias input signal and outputs an optical frequency signal f0. A signal controlled microwave oscillator receives a frequency control input signal and outputs a microwave frequency signal, fm. A single sideband mixer processes the signals output by the laser oscillator and microwave oscillator to generate a signal controlled optical frequency signal.
The single sideband mixer may comprise an optical single sideband mixer that includes a zero degree power splitter coupled to an output of the signal controlled laser oscillator, and a ninety degree hybrid coupler coupled to an output of the signal controlled microwave oscillator. First and second optical modulators having first inputs are coupled to first outputs of the power splitter and hybrid coupler, and second inputs are coupled to second outputs of the power splitter and hybrid coupler. An output ninety degree hybrid coupler is coupled to outputs of the first and second optical modulators for outputting upper sideband and lower sideband output signals.